U.S. Pat. No. 4,953,960 discloses a catadioptric reduction objective of the kind referred to above. This objective includes four lens groups with the third lens group being arranged between the beam splitter and the concave mirror. This arrangement is intended to correct low order coma and spherical aberration of the mirror as well as the Gauss errors. The arrangement of the lens groups between the beam splitter and the mirror and the two-time passage of light resulting therefrom require however a high tolerance sensitivity to decentering with an influence on the coma correction. The refractive power of this third lens group is almost zero in order not to endanger a broad-band spectral correction.
For a significant increase of the image-side aperture, the space requirements of this third lens group require that the fourth lens group be spaced a very great distance from the mirror which, in an extreme case, can be greater than the focal length of this fourth lens group. In addition to a considerably greater manufacturing complexity, this also leads to a large increase in the difficulties encountered with respect to optical correction.
U.S. Pat. No. 5,089,913, in FIGS. 6, 10, 11, 12 and tables 2 to 5, discloses similar objectives having one spherical lens surface between the beam splitter and the concave mirror, either realized as a planoconvex lens fitted directly to the beam-splitting cube (FIGS. 6 and 10), or as a Mangin mirror (FIGS. 11 and 12).
All examples have a numerical aperture of 0.6. The reduction ratio is 5 (FIG. 6) and 4 for the others. The retrofocus ratio of the image-side lens group is 0.98 (FIG. 6), 1.01 (FIG. 10), 0.934 (FIG. 11) and 0.95 (FIG. 12). The free working distances (distance between last lens surface and target) are 0.5, 0.5991, 0.1933 and 0.1903 mm, respectively.
The schematic of FIG. 1 and the example of FIG. 2 and table 1 show objectives without a lens between the beam splitter and the mirror.
However, the geometry of FIG. 2 is obviously incompatible with the data of table 1 and the data of table 1 have been tried with a lens calculation algorithm and have proven not to be utilizable as starting values for such an objective. Hence, it is not possible for the specialist in microlithographic lens design to learn more from this than the contents of the schematic of FIG. 1, namely that a microlithographic catadioptric reduction objective might in principle be possible without a lens between the beam splitter and the mirror. However, nothing is to be found about the conditions for this possibility.
U.S. Pat. No. 3,698,808 (see claim 6, FIG. 4) shows a microlithographic projection apparatus with a first lens group, a semireflecting plane mirror arranged under 45 degrees, a concave mirror and a second lens group arranged under 90 degrees with respect to the axis of the first lens group and the concave mirror.
The separation of the projection objective and the introduction of the semireflecting plane mirror serves the introduction of a second, visible, light source for position control. The reduction ratio is minus one, both lens groups having the same high aperture and the mirror giving no reduction. It is difficult to transform such a construction to the demands of an objective with relevant reduction, as known, for example, from the introduction of U.S. Pat. No. 4,953,960.